Methyltertiarybutylether (MTBE) is well recognized as a suitable blending stock for increasing the octane rating of gasoline. This material has been in use for sometime in Europe as a blending component with gasoline and the usage of MTBE in the United States is increasing. MTBE and related ethers, blended into gasoline at a 10 to 20 percent level, increase both motor and research octane numbers by several units.
MTBE is produced by reacting isobutylene with methanol. The reaction normally is conducted in the liquid phase and under relatively mild conditions. While mixed butylene streams can be used, only the tertiary olefin, isobutylene, reacts at the conditions employed. The catalyst normally used is an ion-exchange resin. The isobutylene component can be obtained from various sources such as naphtha cracking or catalytic cracking. Typically in a refinery a mixture of C.sub.4 's (butanes-butenes) would be utilized as feedstock to the MTBE unit, with the unreacted C.sub.4 's leaving the unit preferably being used as feed to an HF or sulfuric acid alkylation unit.
In HF alkylation it is known that too much water in the HF catalyst can adversely affect the alkylation reaction and can present problems of equipment corrosion. In addition, methanol is detrimental in the HF alkylation since methanol reacts with isobutane to produce unwanted, low octane, high volatility five carbon atom hydrocarbons and water, which undesirably dilutes the HF catalyst. Although to a lesser degree, materials such as methanol and MTBE also have a detrimental effect on the sulfuric acid alkylation. In view of this, methanol which remains in the C.sub.4 's after fractionation for removal of the MTBE product must be removed prior to using the C.sub.4 's in the alkylation step.
Conventional technology uses liquid-liquid extraction to remove methanol from the C.sub.4 hydrocarbon stream (with water as the solvent), and fractionation of methanol from the water. Water is recycled to the extractor, and methanol is recycled to the MTBE reactors. Other schemes described in the art utilize an adsorbent to remove methanol from the C.sub.4 hydrocarbon stream. Various C.sub.4 's from within the MTBE unit are used to recover methanol by desorption for recycle to the MTBE reaction zone. When methanol contained in C.sub.4 's is recycled to the MTBE reaction, control of fresh methanol to the process must be corrected to allow for varying rates of recycle methanol recovered from the adsorbers. The process is then unsteady state. If the fresh methanol rate is not adequately reduced to account for recycle methanol, loading to the adsorption system is increased, potentially to the extent that capacity of the system becomes inadequate. If the fresh methanol feed rate is overcompensated for recycle, isobutene conversion, and hence MTBE production, is unnecessarily reduced. The unsteady state problem can be solved by introducing surge capacity into the system but the large surge capacity requirements increase the capital cost of the process.
In the method of this invention, an adsorbent is used to recover methanol and unreacted C.sub.4 's from the MTBE process. Methanol is desorbed in a closed loop regeneration system in which a circulating vapor is passed through the adsorbent at an elevated temperature. Desorbed methanol exits with the circulating vapor which is cooled to condense the methanol. Liquid methanol is removed from the system and recycled to the MTBE reaction. The purified C.sub.4 stream provides an excellent feedstock for the HF alkylation reaction.
The method of this invention recovers methanol in sufficiently high purity that only minimal liquid surge capacity is needed to allow the methanol to be recycled at a constant flow rate to the MTBE reaction. This eliminates unsteady state control problems and allows a minimum size of adsorption system for the maximum production of MTBE. The unreacted C.sub.4 's stream recovered from the adsorption step is water-free so that it does not need to be dried prior to entering the alkylation unit. In contrast if conventional water wash is used to remove methanol, the C.sub.4 stream is saturated with water and requires dehydration prior to alkylation.